1. Field of the Invention
The present invention relates generally to satellite communication and navigation and, in particular, to a method for utilizing GPS and crosslink signals in distributed spacecraft systems to correct for ionospheric errors in space navigation solutions.
2. Description of the Related Art
Distributed spacecraft systems (e.g. distributed satellite systems or spacecraft systems with some type of propulsion device) use multiple spacecraft to augment the capabilities of monolithic space system approaches. These systems, also referred to as formation flying systems, enable complex sensing tasks such as distributed aperture processing, co-observation, multipoint observation, and distributed interferometry, which are beyond the abilities of single spacecraft systems. Depending on the degree of inherent coordination, formation-flying systems differ from traditional satellite constellations in that the distributed system is treated as a whole, unified by common objectives. Both the National Aeronautics and Space Administration (NASA) and the Department of Defense (DoD) have identified distributed spacecraft systems as a means to achieve mission goals in future deployments. NASA, for example, has identified campaigns of several space missions that largely rely on multiple spacecraft deployments. Operationally, such systems are in their infancy.
A significant number of Earth and space science goals rely on the successful deployment and operation of distributed spacecraft technology within future operational missions. In conjunction with fundamental science, distributed spacecraft military missions in support of defense operations have been identified as important capabilities to maintain national interests.
The specific advantages attributed to the use of distributed spacecraft systems include increased capability, gradual performance degradation (as opposed to catastrophic failure) in that failure of one of the spacecraft does not render the system obsolete, improved system robustness, and overall long term cost efficiency. Relative to single spacecraft systems, formation-flying systems provide improved capability by spatially disbursing sensors, thereby supporting extended and adaptive baselines for distributed sensing tasks. This approach also supports temporal sampling at variable resolutions and is a systematic mechanism for implementing space-based multi-sensor data fusion systems. Because capability is distributed among multiple spacecraft such as satellites, re-deploying functioning spacecraft can mitigate failures that impact individual spacecraft. Thus, while performance in terms of resolution or coverage of a target area may be reduced due to diminished spacecraft, basic functionality is retained. Compensating for failures in this manner allows distributed spacecraft systems to realize an improved level of robustness beyond that of a single spacecraft approach. Finally, the goal of cost-efficiency is embodied in the fact that such formation flying systems rely on the collective faculties of multiple, individually limited spacecraft. This often necessitates the use of small, economical spacecraft approaches that can be deployed in clusters to reduce launch costs.
Realizing the advantages of distributed spacecraft systems, however, entails considerable complexity in system design and implementation. It is not simply by virtue of the fact that multiple spacecraft may be deployed that advantageous performance, capability, robustness, or cost efficiencies can be achieved. For coordinated formations, particularly autonomous or coherent distributed spacecraft systems, technologies and methodologies must provide mechanisms to support information exchange, coordination, autonomy, and dynamic adaptivity. The ability to realize such characteristics in a system must be greater than or commensurate with the level of coordination that is desired within the formation. For example a loosely coupled, non-coherent system may only require crosslink communications to exchange state information or support health and status sharing among spacecraft. Alternatively, a coherent system designed to act as a distributed aperture (e.g., a virtual spacecraft with distributed elements) would require a considerably higher level of distributed control, precision navigation, precision differential timing, and high-rate crosslink communications for coordination and science data exchange.
Supporting collective system operations, coordination, and science among distributed spacecraft necessitates functionality in navigation, communications, and control that leverage complex interactions among spacecraft and between spacecraft and the operating environment. A system that addresses these functions in an integrated, modular manner and that provides a structured approach to distributed spacecraft system design and implementation to effectively realize the advantages of such a system, has been disclosed in an application entitled “Integrated Navigation And Communication System For Use In Distributed Spacecraft Systems” filed by applicants on Jun. 14, 2002 in the United States Patent and Trademark Office and assigned Application Ser. No. 10/172,018, the contents of which are incorporated by reference herein.
FIG. 1 is a conceptual model of the integrated navigation and communication system illustrating the interdependence of system functionality and the capabilities that those functions support. The coordination of distributed autonomous systems such as formation flying spacecraft is typically defined as control in conjunction with communication among non-co-located spacecraft. Coordination is achieved by the perception of the system state and the identification of events that impact that system state. As a distributed system, coordination requires both local and global knowledge and thus communication of applicable information among spacecraft is vital. Control, in conjunction with navigation, forms the foundation for system operations needed to take advantage of the distribution of spacecraft.
Because a basic motivation for deploying distributed spacecraft systems is the ability to support and adapt the spatial dissemination of sensors, such systems require knowledge of spacecraft state as well as generated control actions to effect state changes. Performing scientific tasks also requires the ability to communicate information among spacecraft for on-board processing such as data alignment, data correlation, and data fusion.
FIG. 2 is block diagram illustrating an integrated navigation and communication system 200 for multiple distributed spacecraft flying in formation, hereinafter referred to as a Crosslink Transceiver (CLT). As such, the CLT directly supports the implementation of the fundamental functions required to enable distributed spacecraft systems, including absolute and relative navigation, interspacecraft communications, and autonomous event detection for distributed command and control. Relative navigation, a fundamental measurement for data alignment and data correlation among distributed spacecraft, is determined through a variety of methods to support broad classes of formation flying missions. However, when performing relative navigation in a near-Earth environment, a primary error source is ionospheric distortion on ranging signals, which consequently induce errors in range and range rate estimates.
As stated above, ranging signals are significantly affected by their passage through the ionosphere. The ionosphere is a layer of the atmosphere at an elevation of 150-1,000 km that contains free electrons generated by ionizing radiation from the sun. The distribution and density of the free electrons at a given point in the ionosphere varies strongly with the time of day, the time of year, and the state of the solar sunspot cycle. There is also a significant unpredictable variation due to fluctuations in solar activity. The ionosphere can typically delay microwave signals from satellites by up to 100 ns. Time signals generated by GPS satellites are highly accurate and stable. Thus, a GPS receiver can potentially be used as a simple, low-cost local time standard. However, the unpredictability and variability of the ionospheric delay limits the accuracy and stability of the time that can be generated by a conventional GPS receiver.
GPS receivers exist that are capable of detecting microwave signals at both of the two widely separated frequencies, L1 and L2, transmitted by the GPS satellites. Currently, such receivers remove the effect of the ionospheric delay using the fact that the ionosphere imposes a group delay on the microwave signals that varies inversely with the square of the carrier frequency. However, the L2 ranging signal is currently encrypted and cannot easily be decoded by users who are not qualified by the United States Department of Defense. Some two-frequency receivers are available for non-qualified users, but they are considerably more expensive, and somewhat less reliable, than single-frequency receivers. Accordingly, many users have single-frequency receivers that can receive only the L1 signal.
Commonly, single-frequency receivers usually include a correction for the ionospheric delay based on an ionosphere model that is built into the GPS system. This model is expected to remove about 50% of the ionosphere effect, on average. Since the parameters of the model are estimated in advance and are then transmitted to the GPS satellites, they cannot anticipate day-to-day random fluctuations, and cannot be completely accurate. Alternatively, various organizations make detailed and accurate models of the ionosphere based on GPS observations. However, these models are not available simply, or in real-time.
Therefore, a need exists for a method of measuring the ionospheric delay of a signal transmitted through the ionosphere, enabling single-frequency GPS receivers and crosslink signals in distributed spacecraft systems to correct for ionospheric errors in space navigation solutions, specifically, for relative navigation.